THEORETICAL INVESTIGATIONS OF THE ABLATION OF A GLASS-TYPE HEAT PROTECTION SHIELD OF VARIED MATERIAL PROPERTIES AT THE STAGNATION POINT OF A RE-ENTERING IRBM

摘要

The melting-type heat protection at the stagnation point of a re-entering IRBM is treated by employing homogeneous, opaque, and nondecomposing glass shields which do not exceed a temperature of 460°K at their thermally insulated inner edges. This report describes some effects due to variations of the glass properties. The ballistic re-entry vehicle has a nose diameter of 0.635 m , a ballistic factor of 3.5 x 10-³ m³/kg sec² , a re-entry angle of 124.9° (from the vertical) at an altitude of 100 km , and a re-entry speed of 4.5nkm/sec . The performance of 36 different glass shields with assumed combinations of material properties is investigated by employing a calculation method which yields practically exact, transient solutions for the problem. As a corollary, results for a certain steady flight state are also given. The discussions made it possible to derive under realistic flight conditions some thermal characteristics for the employment of thin, or light-weight, glass shields.nInvestigation of these hypothetical glass shields leads to the conclusion that a low thermal conductivity and a high specific heat, and thus, a small thermal diffusivity are most desirable. A small thermal diffusivity yields high surface temperatures, causing a high radiative heat transfer out of the shield;and steep temperature profiles normal to the surface, causing a small thermal penetration across the shield with little total ablation of the shield. Results show that for the assumed IRBM re-entry, the necessary thickness of the employed glass shields increases monotonically with thermal diffusivity which is the only material parameter affecting this thickness.nA high viscosity level and a high emissivity constant of the sur¬face of the supposedly opaque shield are also desirable;although, these two properties exert a comparatively small influence on the overall performance when disregarding glass shields with an extremely low viscosity level.